This graph tracks changes in the dates
that oak trees in southeastern England first show their leavesone
measure of the start of spring. Data recorded between 1746
and 1958 by Norfolks Marsham family are in blue. Data
from Jean Combes, who recorded leafing dates in Surrey from
1947 to the present, are in red.

For oaks in England, it is generally true that the warmer
the temperature is, the earlier the trees leaf. On this graph,
the overall trend shows average leafing dates (represented
by the smooth line) advancing by several days over the period
shown.

Phenology is the study of the timing of natural events, like
bird migrations or the blooming of flowers. To find trends
in climate change, researchers often turn to past records
of phenological eventslike these records of oak tree
leafing. But researchers must be cautious about the conclusions
they draw. How reliable is data collected hundreds of years
ago? And how might factors like changes in air pollution and
precipitation offset the trends researchers see?

Notice that the graph includes
peaks and valleys that vary radically from year
to year. Phenomena that are naturally inconsistent
produce what scientists call noisy
data. With so much variation, it can be hard to
see any trends at all. To track long-term changes,
researchers must take averages of the changes
they see. At the same time, they must make sure
that their own biases and expectations dont
influence their interpretation of the data.

related
sites

 The
Woodland Trust - A
site devoted to monitoring changes in the
timing of natural events in England.